Metastasis is responsible for over 90% of breast cancer deaths, however no current treatments directly target metastatic cancer. Results from decades of treatment of metastatic breast cancer patients has been bleak, with studies indicating only slight increases in survival which is often accompanied by the severe side effects of systemic chemotherapy. Recent studies have revealed that the mechanical properties of cellular microenvironments (mechano-niche), specifically stiffness, have a close relation with breast cancer metastasis. Specifically, it has been noted that the breast tumor secretes enzymes (lysyl oxidase) into the circulatory system that crosslink collagens and increase the stiffness of tissues accumulate at areas in the lung where metastases occur. Additionally, mesenchymal stem cells (MSCs) can actively and selectively home to breast cancer metastases when injected into the blood and their differentiation is tightly regulated by the stiffness of the local environment. In light of the tight correlation between tissue stiffness with breast cancer metastasis and MSC differentiation, I propose to develop a mechanoresponsive cell system (MRCS) to directly target the mechano-environmental cues of breast cancer metastases for localized and specific delivery of diagnostic reporters and anti-tumor agents. The project will involve 1) establishing the MRCS by identifying and cloning stiffness-specific promoters in MSCs. These promoters can be used to drive expression of imaging probes and therapeutics, 2) testing the ability of MRCS to detect cancer metastases and measure stiffness in metastatic niche in vivo, and 3) treating breast cancer lung metastases in vivo using MRCS through specific and local activation of therapeutics. Upon accomplishing the above goals, we will have created the first therapeutic system to directly interrogate matrix stiffness and applied it to localized delivery of agents to breast cancer metastases. This system has major clinical implications in increasing the effectiveness of therapies for the over 150,000 Americans living with metastatic breast cancer while also ameliorating the symptoms associated with systemic chemotherapy. Importantly, the proposed MRCS will serve as a platform technology for future application to therapies targeting aberrant tissue stiffness in potentially any types of cancer metastasis. Additionally, our MRCS has the potential to become a routine practice for diagnosis of micrometastases and for monitoring treatment in high-risk patient groups. Furthermore, generation of a cell-based stiffness sensor for measurement of matrix mechanical properties in situ will represent a paradigm-shifting method of dynamically interrogating the mechano-environment of primary tumors, metastases, and changes in matrix stiffness during disease progression and response to therapies at a cellular resolution in vivo.